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[Version 10.0] Feature support for improved definite assignment #1545

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68803ac
Support improved definite assignment
RexJaeschke Jan 23, 2026
a675260
support improved definite assignment
RexJaeschke Jan 23, 2026
88c8e46
fix md spacing
RexJaeschke Jan 23, 2026
fd56537
fix link
RexJaeschke Jan 23, 2026
3fa8a5c
review and grammar fixes
BillWagner Mar 24, 2026
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7 changes: 7 additions & 0 deletions standard/expressions.md
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An expression is a sequence of operators and operands. This clause defines the syntax, order of evaluation of operands and operators, and meaning of expressions.

An expression *E* is said to ***directly contain*** a subexpression *E₁* if it is not subject to a user-defined conversion [§10.5](conversions.md#105-user-defined-conversions) whose parameter is not of a non-nullable value type, and one of the following conditions holds:

- *E* is *E₁*.
- If *E* is a parenthesized expression `(E₂)`, and *E₂* directly contains *E₁*.
- If *E* is a null-forgiving operator expression `E₂!`, and *E₂* directly contains *E₁*.
- If *E* is a cast expression `(T)E₂`, and the cast does not subject *E₂* to a non-lifted user-defined conversion whose parameter is not of a non-nullable value type, and *E₂* directly contains *E₁*.

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@RexJaeschke RexJaeschke Jan 23, 2026

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The MS proposal had this text in the new section, 9.4.4.3x ?.expressions, but as this text is general, and not specific to that expression form, I thought it did not belong there. Short of creating a new section just for this text, I put it in 12.1. Is that OK?

## 12.2 Expression classifications

### 12.2.1 General
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> *Example*: For the operation `b * s`, where `b` is a `byte` and `s` is a `short`, overload resolution selects `operator *(int, int)` as the best operator. Thus, the effect is that `b` and `s` are converted to `int`, and the type of the result is `int`. Likewise, for the operation `i * d`, where `i` is an `int` and `d` is a `double`, `overload` resolution selects `operator *(double, double)` as the best operator. *end example*


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standard/expressions.md:301 MD012/no-multiple-blanks Multiple consecutive blank lines [Expected: 1; Actual: 2] https://github.com/DavidAnson/markdownlint/blob/v0.40.0/doc/md012.md
**End of informative text.**

#### 12.4.7.2 Unary numeric promotions
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#### 9.4.4.24 Invocation expressions and object creation expressions

If the method to be invoked is a partial method that has no implementing partial method declaration, or is a conditional method or conditional local function for which the call is omitted ([§23.5.3.2](attributes.md#23532-conditional-methods), §conditional-local-function), then the definite-assignment state of *v* after the invocation is the same as the definite-assignment state of *v* before the invocation. Otherwise the following rules apply:

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For an invocation expression *expr* of the form:

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- The definite-assignment state of *v* before *expr_second* is the same as the definite-assignment state of *v* after *expr_first*.
- The definite-assignment statement of *v* after *expr* is determined by:
- If *expr_first* is a constant expression ([§12.26](expressions.md#1226-constant-expressions)) with value `null`, then the state of *v* after *expr* is the same as the state of *v* after *expr_second*.
- If *expr_first* directly contains ([§12.1](expressions.md#121-general)) a null-conditional expression *E*, and *v* is definitely assigned after the non-conditional counterpart *E₀* (§qdot-expressions), then the definite-assignment state of *v* after *expr* is the same as the definite-assignment state of *v* after *expr_second*.
- Otherwise, the state of *v* after *expr* is the same as the definite-assignment state of *v* after *expr_first*.

> *Note*: The rule above formalizes that for an expression like `a?.M(out x) ?? (x = false)`, either the `a?.M(out x)` was fully evaluated and produced a non-null value, in which case `x` was assigned, or the `x = false` was evaluated, in which case `x` was also assigned. Therefore `x` is always assigned after this expression.
>
> This also handles the `dict?.TryGetValue(key, out var value) ?? false` scenario, by observing that *v* is definitely assigned after `dict.TryGetValue(key, out var value)`, and *v* is “definitely assigned when true” after `false`, and concluding that *v* must be “definitely assigned when true.”
>
> The more general formulation also allows the handling of some more unusual scenarios, such as:
>
> - `if (x?.M(out y) ?? (b && z.M(out y))) y.ToString();`
> - `if (x?.M(out y) ?? z?.M(out y) ?? false) y.ToString();`
>
> *end note*

#### 9.4.4.30 ?: expressions

For an expression *expr* of the form:
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- Otherwise, if *expr_cond* is a constant expression ([§12.26](expressions.md#1226-constant-expressions)) with value `false` then the state of *v* after *expr* is the same as the state of *v* after *expr_false*.
- Otherwise, if the state of *v* after *expr_true* is definitely assigned and the state of *v* after *expr_false* is definitely assigned, then the state of *v* after *expr* is definitely assigned.
- Otherwise, the state of *v* after *expr* is not definitely assigned.
- If the state of *v* after *expr_true* is “definitely assigned when true,” and the state of *v* after *expr_false* is “definitely assigned when true,” then the state of *v* after *expr* is “definitely assigned when true.”
- If the state of *v* after *expr_true* is “definitely assigned when false,” and the state of *v* after *expr_false* is “definitely assigned when false,” then the state of *v* after *expr* is “definitely assigned when false.”
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@RexJaeschke RexJaeschke Jan 23, 2026

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The instructions in the MS spec for including the new bullets are unclear. They appear to suggest adding the new bullets at the end of the level-2 bullet list. However, that would make them come after the currently final “Otherwise” bullet, which doesn’t seem right. But that’s where I put them. Please check if that makes sense.


> *Note*: When both arms of a conditional expression result in a conditional state, the corresponding conditional states are joined and propagated out instead of unsplitting the state and allowing the final state to be non-conditional. This enables scenarios like the following:
>
> ```csharp
> bool b = true;
> object x = null;
> int y;
> if (b ? x != null && Set(out y) : x != null && Set(out y))
> {
> y.ToString();
> }
>
> bool Set(out int x) { x = 0; return true; }
> ```
>
> *end note*

#### 9.4.4.31 Anonymous functions

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- If the variable ‘v’ is declared in *pattern*, then the definite-assignment state of ‘v’ after *expr* is “definitely assigned when true”.
- Otherwise the definite assignment state of ‘v’ after *expr* is the same as the definite assignment state of ‘v’ after *expr_operand*.

#### §qdot-expressions ?. expressions

For an expression *E* of the form:

```csharp
«primary_expression» ?. «null_conditional_operation»
```

let *E₀* be the expression obtained by textually removing the leading `?` from each of the *null_conditional_operation*s of *E* that have one. (*E₀* is referred to as the ***non-conditional counterpart*** to the null-conditional expression.)

- The definite-assignment state of *v* at any point within *E* is the same as the definite-assignment state at the corresponding point within *E₀*.
- The definite-assignment state of *v* after *E* is the same as the definite-assignment state of *v* after *primary_expression*.

> *Note*: *null_conditional_operation* is not actually a grammar rule; rather, it represents any form permitted by the grammar at that location. It is used here for convenience. *end note*
<!-- markdownlint-disable MD028 -->

<!-- markdownlint-enable MD028 -->
> *Note*: The concept of “directly contains” allows skipping over relatively simple “wrapper” expressions when analyzing conditional accesses that are compared to other values. For example, in general, `((a?.b(out x))!) == true` is expected to result in the same flow state as `a?.b == true`.
>
> The intent is to allow analysis to function in the presence of a number of possible conversions on a conditional access. Propagating out “state when not null” is not possible when the conversion is user-defined, though, since one can't count on user-defined conversions to honor the constraint that the output is non-null only if the input is non-null. The only exception to this is when the user-defined conversion’s input is a non-nullable value type. For example:
>
> ```csharp
> public struct S1 { }
> public struct S2 { public static implicit operator S2?(S1 s1) => null; }
> ```
>
> This also includes lifted conversions like the following:
>
> ```csharp
> string x;
>
> S1? s1 = null;
> _ = s1?.M1(x = "a") ?? s1.Value.M2(x = "a");
>
> x.ToString(); // ok
>
> public struct S1
> {
> public S1 M1(object obj) => this;
> public S2 M2(object obj) => new S2();
> }
> public struct S2
> {
> public static implicit operator S2(S1 s1) => default;
> }
> ```
>
> When it is considered whether a variable is assigned at a given point within a null-conditional expression, it can simply be assumed that any preceding null-conditional operations within the same null-conditional expression succeeded.
>
> For example, given a conditional expression `a?.b(out x)?.c(x)`, the non-conditional counterpart is `a.b(out x).c(x)`. If the definite-assignment state of `x` before `?.c(x)` is to be determined, for example, a “hypothetical” analysis of `a.b(out x)` can be performed and the resulting state can be used as an input to `?.c(x)`. *end note*

#### §boolean-constant-expressions Boolean constant expressions

For an expression *expr*, where *expr* is a constant expression with a `bool` value, the definite-assignment state of *v* after *expr* is determined, as follows:

- If *expr* is a constant expression with value *true*, and the state of *v* before *expr* is “not definitely assigned,” then the state of *v* after *expr* is “definitely assigned when false.”
- If *expr* is a constant expression with value *false*, and the state of *v* before *expr* is “not definitely assigned,” then the state of *v* after *expr* is “definitely assigned when true.”

> *Note*: It is assumed that if an expression has a constant value bool `false`, that it's impossible to reach any branch that requires the expression to return `true`. Therefore, variables are assumed to be definitely assigned in such branches.
>
> Being in a conditional state *before* visiting a constant expression, is never expected, so there is no need to account for scenarios such as “*expr* is a constant expression with value *true* and the state of *v* before *expr* is definitely assigned when true.” *end note*

#### §relational-equality-expressions ==/!= expressions

For an expression *expr* of the form:

```csharp
«expr_first» == «expr_second»
```

where `==` is a predefined comparison operator ([§12.15](expressions.md#1215-relational-and-type-testing-operators)) or a lifted operator ([§12.4.8](expressions.md#1248-lifted-operators)), the definite-assignment state of *v* after *expr* is determined by:

- If *expr_first* directly contains ([§12.1](expressions.md#121-general)) a null-conditional expression *E* and *expr_second* is a constant expression with value `null`, and the state of *v* after the non-conditional counterpart *E₀* is “definitely assigned,” then the state of *v* after *expr* is “definitely assigned when false.”
- If *expr_first* directly contains a null-conditional expression *E* and *expr_second* is an expression of a non-nullable value type, or a constant expression with a non-null value, and the state of *v* after the non-conditional counterpart *E₀* is “definitely assigned,” then the state of *v* after *expr* is “definitely assigned when true.”
- If *expr_first* is of type `bool`, and *expr_second* is a constant expression with value `true`, then the definite-assignment state after *expr* is the same as the definite-assignment state after *expr_first*.
- If *expr_first* is of type `bool`, and *expr_second* is a constant expression with value `false`, then the definite-assignment state after *expr* is the same as the definite-assignment state of *v* after the logical negation expression `!`*expr_first*.

For an expression *expr* of the form:

```csharp
«expr_first» != «expr_second»
```

where `!=` is a predefined comparison operator ([§12.15](expressions.md#1215-relational-and-type-testing-operators)) or a lifted operator ([§12.4.8](expressions.md#1248-lifted-operators)), the definite-assignment state of *v* after *expr* is determined by:

- If *expr_first* directly contains a null-conditional expression *E* and *expr_second* is a constant expression with value `null`, and the state of *v* after the non-conditional counterpart *E₀* is “definitely assigned,” then the state of *v* after *expr* is “definitely assigned when true.”
- If *expr_first* directly contains a null-conditional expression *E* and *expr_second* is an expression of a non-nullable value type, or a constant expression with a non-null value, and the state of *v* after the non-conditional counterpart *E₀* is “definitely assigned,” then the state of *v* after *expr* is “definitely assigned when false.”
- If *expr_first* is of type `bool`, and *expr_second* is a constant expression with value `true`, then the definite-assignment state after *expr* is the same as the definite-assignment state of *v* after the logical negation expression `!`*expr_first*.
- If *expr_first* is of type `bool`, and *expr_second* is a constant expression with value `false`, then the definite-assignment state after *expr* is the same as the definite-assignment state after *expr_first*.

All of the above rules are commutative.

> *Note*: The general idea expressed by these rules is:
>
> - if a conditional access is compared to `null`, then the operations definitely occurred if the result of the comparison is `false`.
> - if a conditional access is compared to a non-nullable value type or a non-null constant, then the operations definitely occurred if the result of the comparison is `true`.
> - since user-defined operators can’t be trusted to provide reliable answers where initialization safety is concerned, the new rules only apply when a predefined `==`/`!=` operator is in use.
>
> Some consequences of these rules are:
>
> - `if (a?.b(out var x) == true) x() else x();` will error in the 'else' branch
> - `if (a?.b(out var x) == 42) x() else x();` will error in the 'else' branch
> - `if (a?.b(out var x) == false) x() else x();` will error in the 'else' branch
> - `if (a?.b(out var x) == null) x() else x();` will error in the 'then' branch
> - `if (a?.b(out var x) != true) x() else x();` will error in the 'then' branch
> - `if (a?.b(out var x) != 42) x() else x();` will error in the 'then' branch
> - `if (a?.b(out var x) != false) x() else x();` will error in the 'then' branch
> - `if (a?.b(out var x) != null) x() else x();` will error in the 'else' branch
>
> *end note*

#### §isop-ispattern-expressions is operator and is pattern expressions
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@BillWagner BillWagner Mar 24, 2026

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I'm curious how this differs from 9.4.4.34 (is pattern expression). Should these two clauses be merged? If not, we need text explaining why they are different.


For an expression *expr* of the form:

```csharp
«E» is «T»
```

where *T* is any type or pattern:

- The definite-assignment state of *v* before *E* is the same as the definite-assignment state of *v* before *expr*.
- The definite-assignment state of *v* after *expr* is determined by:

- If *E* directly contains ([§12.1](expressions.md#121-general)) a null-conditional expression, and the state of *v* after the non-conditional counterpart *E₀* is “definitely assigned,” and `T` is any type or a pattern that does not match a `null` input, then the state of *v* after *expr* is “definitely assigned when true.”
- If *E* directly contains a null-conditional expression, and the state of *v* after the non-conditional counterpart *E₀* is “definitely assigned,” and `T` is a pattern that matches a `null` input, then the state of *v* after *expr* is “definitely assigned when false.”
- If *E* is of type `bool` and `T` is a pattern that only matches a `true` input, then the definite-assignment state of *v* after *expr* is the same as the definite-assignment state of *v* after *E*.
- If *E* is of type `bool` and `T` is a pattern that only matches a `false` input, then the definite-assignment state of *v* after *expr* is the same as the definite-assignment state of *v* after the logical negation expression `!`*expr*.
- Otherwise, if the definite-assignment state of *v* after *E* is "definitely assigned," then the definite-assignment state of *v* after *expr* is "definitely assigned."

> *Note*: This subclause addresses similar scenarios as §relational-equality-expressions. It does not, however, address recursive patterns; e.g., `(a?.b(out x), c?.d(out y)) is (object, object)`. *end note*

## 9.5 Variable references

A *variable_reference* is an *expression* that is classified as a variable. A *variable_reference* denotes a storage location that can be accessed both to fetch the current value and to store a new value.
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